1. Field of the Invention
The present invention relates to a fuel injection control system having a split fuel injection mode for a diesel engine.
2. Description of the Prior Art
Heretofore, diesel engines are controlled by a fuel injection control system that determines the quantity of fuel to be injected into an engine cylinder and the time at which the fuel is to be injected into the engine cylinder, based on various engine conditions including the rotational speed of the engine and the accelerator opening, i.e., the throttle position or opening. In the actual control of the fuel injection timing, the fuel injection control system starts injecting the fuel a calculated period of time after a reference crankshaft angular position. For example, as shown at [PHASE A] in FIG. 25 of the accompanying drawings, if the fuel injection is to begin at a time TFIN, then the fuel injection control system, taking into account a response time delay TD of a fuel injection valve, produces a drive pulse to actuate the fuel injection valve a period of time TT after a signal 0 indicative of the reference crankshaft angular position. The period of time TT, which is to vary with the engine rotational speed Ne, is determined so that it is shorter as the engine rotational speed Ne is higher, using the engine rotational speed Ne detected for the fuel injection into a preceding cylinder. In this manner, the fuel injection control system starts injecting the fuel into the cylinder accurately at a desired crankshaft angular position. The drive pulse occurs continuously for a period of time TQ which depends upon the quantity of fuel to be injected.
When the engine is cranked at startup, the engine rotational speed increases greatly and hence varies to a large extent. Therefore, when the engine is cranked, determining the period of time TT based on the engine rotational speed detected for the fuel injection into a preceding cylinder fails to eject the fuel at a desired crankshaft angular position. As a result, the engine may not be started quickly.
FIGS. 24 and 25 of the accompanying drawings show how the engine rotational speed Ne varies upon cranking and the manner in which the fuel injection timing is controlled. In FIG. 24, the engine is cranked at an engine rotational speed of 150 rpm. At [PHASE A], the injected fuel is successfully ignited for explosion. Thereafter, the engine rotational speed Ne increases quickly, and the injected fuel fails to be ignited at [PHASE B], resulting in a misfire. The misfire is caused because, as shown at [PHASE B] in FIG. 25, the actual fuel injection timing is delayed by ID from the target fuel injection timing TFIN as a period of time TT' after which the fuel is to be injected is longer than the period of time TT due to a lower engine rotational speed detected for the fuel injection into a preceding cylinder.
When the injected fuel cannot easily be ignited under certain conditions such as cold engine startup, the above fuel injection timing error tends to cause misfires often, making it difficult for the engine to start quickly and smoothly.
FIG. 26 of the accompanying drawings shows intervals of time in which the fuel can be ignited at lower and higher temperatures. In FIG. 26, at lower temperatures, the temperature in the combustion chamber does not rise sufficiently even in the compression stroke, and an ignitable period TUPL at cold engine startup is shorter than an ignitable period TUPH at warm engine startup. At cold engine startup, therefore, it is necessary to inject the fuel into the combustion chamber exactly in the ignitable period TUPL.
For allowing the diesel engines to start quickly and smoothly, it is important that the fuel be injected at an accurate crankshaft angular position. It has however been difficult at engine startup to inject the fuel accurately with ideal timing because the engine rotational speed is low and varies in a wide range.
It is known that some diesel engines incorporate a pilot fuel injection mode in which a smaller quantity of fuel is first injected and ignited for slow burning, and then a larger quantity of fuel in injected and ignited for explosive burning. There have been made certain efforts to improve engine startup with such a pilot fuel injection process. For example, Japanese laid-open utility model publication No. 61-147371 indicates the effectiveness of the pilot fuel injection for improving diesel engine startup. However, as shown in FIG. 3(B) of this publication, since the interval between pilot and main fuel jets is short, the main fuel jet injected immediately after the pilot fuel jet cannot be ignited unless the pilot fuel jet is ignited. Under some conditions in which the fuel injection cannot be executed exactly at ideal times, as upon engine startup, even the fuel introduced by the pilot fuel jet may not be ignited. The disclosed engine startup control device therefore fails to improve the engine startup capability.
Japanese laid-open utility model publication No. 61-147371 also shows a prechamber and a glow plug for heating the prechamber to start the engine quickly particularly at lower temperatures. The disclosed diesel engine is thus relatively complex in structure, and does not produce a high output power compared with diesel engines of the direct injection type.
Another approach for improved engine startup with pilot fuel injection is disclosed in Japanese laid-open patent publication No. 1-227866. According to the disclosed fuel injection device, fuel is applied to a cylinder wall to seal the combustion chamber for improved engine startup. More specifically, a pilot fuel jet is injected before or after the piston reaches the top dead center, so that the injected fuel is applied to a cylinder wall. However, since the pilot fuel jet injected before the piston reaches the top dead center is applied to the cylinder wall, the injected pilot fuel jet does not contribute to preliminary fuel combustion prior to the main fuel jet injection. Furthermore, under certain conditions in which the fuel injection cannot be executed exactly at ideal times, as upon engine startup, the injected main fuel jet may not be ignited. The disclosed fuel injection device therefore fails to improve the engine startup capability either.
As described above, the conventional fuel injection devices suffer fuel injection timing errors at engine startup as the fuel injection timing is determined based on the time that has elapsed from the reference crankshaft angular position. The diesel engines controlled by the conventional fuel injection devices cannot start quickly and smoothly under certain conditions.